Field of the Invention
The present invention relates to an image processing technique of sharpening an image produced by image capturing.
Description of the Related Art
One example of a method of sharpening an image is an unsharp masking process that adds to or subtracts from an original image a difference between an unsharpened image blurred by applying an unsharp mask to the original image and the original image to produce a sharpened image. In this process, an image area having a larger difference between the unsharpened image and the original image is more sharpened.
Japanese Patent Laid-Open No. 2010-081263 discloses a method of applying a one-dimensional correction filter asymmetric with respect to pixel signals arranged in a row in an image height direction (i.e., in a meridional azimuth direction) to reduce an influence of a point spread function (PSF) of an optical system on the pixel signal row.
Furthermore, Japanese Patent Laid-Open No. 2012-073691 discloses a method of reconstructing, depending on coefficient data and information of an image sensor, an optical transfer function (OTF) to be used to produce an image restoration filter.
However, conventional unsharp masking processes uses a rotationally symmetric filter as the unsharp mask and thus have a difficulty in sharpening an image degraded due to an influence of a complex-shaped PSF, such as an asymmetric aberration and a sagittal halo. That is, correcting the aberration generated in one azimuth direction in which the aberration is large generates an undershoot in another azimuth direction in which the aberration is small. Conversely, reducing the undershoot makes it impossible to sufficiently correct the aberration.
The method disclosed in Japanese Patent Laid-Open No. 2010-081263 takes only asymmetry in the image height direction into consideration and uses the one-dimensional correction filter, which makes it impossible to reduce asymmetry in directions other than the image height direction. Furthermore, this method adjusts the asymmetry of the correction filter by changing number of minus tap coefficients and thus cannot deal with a blur due to the PSF of the optical system, which makes it impossible to sufficiently sharpen the image degraded due to the PSF by correcting the degraded image by the correction filter in the image height direction.
When the image restoration filter is used to correct the degraded image, the method disclosed in Japanese Patent Laid-Open No. 2012-073691 enables reconstructing the OTF with less storage amount. The image restoration filter can be produced depending on the OTF. Conversely, since the PSF can be acquired by performing a Fourier transform on the OTF, the method disclosed in Japanese Patent Laid-Open No. 2012-073691 enables converting the reconstructed OTF into the PSF. However, since this method requires, as described above, a process of converting a frequency space into a real space, it is desirable to further reduce a processing load.